Sacculina carcini is member of a highly-specialized group of parasitic cirripeds (Rhizocephala) that use crabs (Carcinus
maenas) as hosts to carry out the reproductive phase of their life cycle. We describe the naupliar development of S.
carcini Thompson, 1836 from a very precise monitoring of three different broods from three specimens. Nauplii were
sampled every 4 h, from the release of the larvae until the cypris stage. Larval development, from naupliar instar 1 to
the cypris stage, lasts 108 h at 18 °C. A rigorous sampling allowed us to describe an additional intermediate naupliar
instar, not described previously. Naupliar instars are renumbered from 1 to 5. Nauplius 1 (N1) larvae hatch in the
interna; N2 are released from the interna and last between 12 and 16 h; N3 appear between 12 and 16 h after release;
N4 appear between 28 and 32 h; and N5 appear between 44 and 48 h. The cypris stage appears between 108 and
112 h. The redescribed morphologies allowed us to identify new characters. Antennular setation discriminates naupliar
instars 3, 4 and 5. Telson and furca morphologies discriminate all naupliar instars. Furthermore, we demonstrate
that the speed of larval development is similar within a single brood and between broods from different specimens,
suggesting synchronization of larval development. From precise monitoring of broods every 4 h, we demonstrate
that the life cycle of S. carcini includes five instars of naupliar larvae instead of four. The morphological characters of
the larvae discriminate these naupliar instars and allow the identification of S. carcini from other Rhizocephala species.
S. carcini larvae develop synchronously. Consequently, they might be an informative model to study larval development